In oil and gas wells and other wells the lining or casing thereof consists of a string of segments of metal tubular members joined by collars. Because this string of casing may span several different zones in the earth, and because it is an electrical conductor it will pass natural electric currents between zones spaced away from the casing from one zone to another zone that is in contact with the casing. The passage of electric currents away from the casing will cause a deterioration of the metal casing due to ionization of the metal and displacement of it into the formation in the form of a chemical compound. When this occurs, material of the casing is displaced and the casing is in effect corroded. If this action continues over a significant period of time the well casing can become perforated or at least not safe for the containment of well pressure. Once the well casing becomes perforated due to this deterioration it can damage the well due to permitting fluid flow into or out of the casing at this unpredicted and probably undesirable location. Leakage such as this can permit well fluids to move between zones and earth formation and not only damage the well but perhaps the surrounding zones in the earth formations by contaminating them with fluids from other zones through which the well passes.
Cathodic protection for a well can overcome the aforementioned problems. This is done by circulating an artificial current between the well and the surrounding earth formation to oppose the type of natural current that causes deterioration of the well's casing. In order to properly apply such a current, it is desirable to determine the amount and polarity of natural current flow so that adequate protection can be employed. Once the amount of natural current flow between the well casing and the earth's formation is determined, this will allow the skilled corrosion engineer to establish criteria for protecting the well by passing an appropriate current to oppose the natural current flow.
Prior to this invention the determination of this natural current flow in the well was done by measuring the voltage at selected points within the well casing and determining the current by using the casing resistance as defined in manufacturer's data of the casing. This prior technique was not particularly satisfactory nor accurate for several reasons. The resistance data available for casing is limited only to new material and thus is not accurate for casing that has been slightly corroded after being within a well. Even new casing materials contains alloy variations and dimensional tolerances which introduce considerable errors into the manufacturer's resistance data. Using these resistance values may be accurate for a newly completed well however they will be inaccurate for a well when the casing corrosion is to be monitored later in the well's life.
It is interesting to note that in older wells the well's owner may not know the type or grade of casing in the well. In this situation, it is not possible to know the resistance of the casing when it was new and it is nearly impossible to estimate what it may be at some point in time many years after the well was completed. Situations such as this render previous current determination techniques unusable.